Neurophysiological, behavioral and anatomical techniques will be used to develop the barn owl as an experimental model for studying sound localization in the auditory system. A special class of auditory neurons in the midbrain and forebrain of the owl have been implicated in the spatial analysis of sound. These neurons are excited by sound only when it originates from a restricted area of space, or receptive field. In one portion of the owl's midbrain these neurons are organized according to the locations of their receptive fields so that they form a map of auditory space. This research proposed to study further the functional characteristics of these specialized neurons and to find the acoustic parameters that determine the borders and properties of their receptive fields. Also the neuro-anatomical pathway associated with these neurons will be traced, and the integration occuring at each step in the discovered pathway will be investigated to determine the sequence of neuronal interactions that finally give rise to receptive fields and the map of auditory space. Simultaneously, I will study the long term effects of monaural hearing loss on sound localizing behavior and space-dependent neuronal properties in the owl. Acute monaural occlusion causes systematic perceptual errors in sound localization. The same occlusion causes neuronal receptive fields to shift their locations. The direction and magnitude of the field shift corresponds to that predicted by the behavioral localization error. The consequences of chronic monaural impairment will be followed behaviorally and neurophysiologically to ascertain if the owl corrects its error with experience, and to elucidate the neuronal correlates of this compensation in the auditory system. The time course of this auditory recalibration and the possibility of its having a critical period will also be investigated.